A right-angled triangle (or right triangle) has one rectangular
angle. Then, Pythagoras' theorem states the relation between the oblique side a and the other two sides
as a2 = b2 + c2.
The triangle inequality relates the three sides in a triangle: a < b + c.

The saw-tooth curve repeats the right
triangle. For equal sides of the right angle we've got of the right
. This triangle
is used in mathematical lessons as 'triangle' or 'set square'. Sometimes this set square
has the form of a special right triangle, a
right triangle with angles of 30 and 60 degrees.

In 1704, the Dominican priest
Sébastien Truchet (1657-1729) published a text 3)
in which he showed that an infinite number of motifs can be used to generate a
pattern.
He considered the possible patterns formed by tilings of (filled) right
isosceles triangles oriented at the four corners of a square: the Truchet
tiling. There are four possible orientations that can be combined,
making beautiful patterns.

The characteristic triangle is the triangle
with infinitesimal sides dx, dy and ds. This 'triangulum characteristicum' has been used
in the 17th century by Pascal, Snellius and Leibniz, while
developing infinitesimal mathematics. It seems that is was Leibniz who introduced
this expression.

In a equilateral triangle (orregular triangle) the three sides have equal
length. These triangles can fill the plane 1). This
quality has already been found by Pythagoras (500 BC). The group of scholars
around Pythagoras had a special liking for numbers, which had for them a magical
meaning. They studied the 'figure numbers' (or 'polygon numbers'), constructed from a
regular polygon.
In the following way, from a regular triangle: and so on.
This results in the triangular numbers 1, 3, 6, 10, 15, and so on.
In Pascal's triangle we find these same
numbers as binomial coefficients (n over k) 2):
1
1
1
1 2
1
1 3 3
1
1 4 6 4
1
1 5 10 10 5
1

These coefficients are often used in probability theory and also in formulae for the nth
power and the nth derivative. In probability theory the triangle diagram, is
used, a way to express the a function of three variables, in a triangle grid.

A curve that can be turned continuously inside an equilateral triangle, is
called a delta curve. There are an infinite
number of these curves.

The Dutch drawer Escher was responsible for the 'impossible triangle': an
impossible (perspective) triangle that can be drawn, however.
When coping with a project, there is a tension between time, money and quality.
You can show this graphically: when you try to diminish the amount of time
spent, that cost you money or quality. This is called the devil's triangle.

What triangular objects we see around us? When going to a symphony orchestra we hear
the triangle, visiting the sea we can find the triangle mussel the triangle shell and the
triangle crab. We have also triangular parts ourselves:

the 'triangular nerve' (the fifth cerebral nerve)

the 'triangular bone': a bone in the back row of carpal bones

the 'triangular muscle': the muscle that goes from the clavicle, shoulder and
shoulder-blade to the outer side of the upper arm

A small pair of trunks or pants is called a triangle (in the Netherlands).

It is rather popular to use the triangular form of geographic regions in their names:

the 'Bermuda triangle' - a sea east of the U.S. that is notorious for his mysterious
disappearances, in apparent good weather conditions 3)

in the 'gold triangle' is the world's largest resource of opium: the borderland of
Thailand, Burma with Laos; the sea-area between Costa Rica, Colombia and Ecuador is also
denoted as a 'golden triangle'.

the 'black triangle' or triangle of death is the extremely polluted borderland of
Poland, Czechia with Germany

the 'iron triangle' is the area northwest of Saigon, where the Vietnam war struck heavy
wounds

the 'brown triangle' is the region of Munich, Nuremberg and Berchtesgarden in Germany,
bastion of the brown-clothed nazis

the 'dark triangle' was a region in Canada, where in 1997 ice-rain made collapse
electricity pylons for 3 million people

'The Triangle' is North Carolina's version of Silicon Valley

in Congo a large area of rainforest is called the Goualogo Triangle

Measuring land for mapping by use of trigonometry is called triangulation.

In the 17th and 18th century traders of the Dutch Republic made 'triangular journeys'.
Goods were transported by ship in three laps. At first Dutch goods as brandy were
transported to western Africa. This was exchanged for slaves who were transported to the
Western Indies. There the slaves were exchanged for goods as cacao, cotton and tobacco, to
be brought home. These journeys took about 15 months each.
The triangle manifests itself also in social behavior: one speaks of the eternal triangle
when there exist intimate relations between three persons. In the Netherlands the regional
cooperation of the Government with the police and the Counsel for the Prosecution is
called the 'regional triangle'.

The (mostly regular) triangle is used as a symbol for certain groups or organizations:

the pink triangle is the symbol for the homosexual movement; its form has been derived
from the German concentration camps, where homosexuals were forced to wear those
triangles.

the red triangle is the symbol of the YMCA

the blue triangle is the symbol of the YWCA

In traffic a triangular sign has always a warning function (a square sign gives a
remark). The warning triangle is also used when your car has a break-down.

At last we know the triangles, for which its quality depends on its position regarding
another triangle. The cyclic triangle, the inscribed triangle,
the circumscribed triangle and the tangent triangle did
we already see at the polygon section.

perspective triangles: two triangles
A1B1C1 and A2B2C2 are
perspective in relation to each other when the lines A1A2, B1B2
en C1C2 meet each other in one point. It was Desargues who
formulated the theory of these phenomena (1648).